尝试使用Djang中的“comments”contrib实现联接时出现问题

Question

我有这个模型，注释是用django_comments控件管理的：

class Fortune(models.Model):
    author = models.CharField(max_length=45, blank=False)
    title = models.CharField(max_length=200, blank=False)
    slug = models.SlugField(_('slug'), db_index=True, max_length=255, unique_for_date='pub_date')
    content = models.TextField(blank=False)
    pub_date = models.DateTimeField(_('published date'), db_index=True, default=datetime.now())
    votes = models.IntegerField(default=0)
    comments = generic.GenericRelation(
        Comment,
        content_type_field='content_type',
        object_id_field='object_pk'
    )

我想检索Fortune对象，为每个对象添加一个nb_comments值，计算它们各自的注释数；我尝试以下查询：

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从外壳上：

>>> from django_fortunes.models import Fortune
>>> from django.db.models import Count
>>> Fortune.objects.annotate(nb_comments=Count('comments'))
[<Fortune: My first fortune, from NiKo>, <Fortune: Another One, from Dude>, <Fortune: A funny one, from NiKo>]
>>> from django.db import connection
>>> connection.queries.pop()
{'time': '0.000', 'sql': u'SELECT "django_fortunes_fortune"."id", "django_fortunes_fortune"."author", "django_fortunes_fortune"."title", "django_fortunes_fortune"."slug", "django_fortunes_fortune"."content", "django_fortunes_fortune"."pub_date", "django_fortunes_fortune"."votes", COUNT("django_comments"."id") AS "nb_comments" FROM "django_fortunes_fortune" LEFT OUTER JOIN "django_comments" ON ("django_fortunes_fortune"."id" = "django_comments"."object_pk") GROUP BY "django_fortunes_fortune"."id", "django_fortunes_fortune"."author", "django_fortunes_fortune"."title", "django_fortunes_fortune"."slug", "django_fortunes_fortune"."content", "django_fortunes_fortune"."pub_date", "django_fortunes_fortune"."votes" LIMIT 21'}

以下是格式正确的sql查询：

SELECT "django_fortunes_fortune"."id", 
       "django_fortunes_fortune"."author", 
       "django_fortunes_fortune"."title", 
       "django_fortunes_fortune"."slug", 
       "django_fortunes_fortune"."content", 
       "django_fortunes_fortune"."pub_date", 
       "django_fortunes_fortune"."votes", 
       COUNT("django_comments"."id") AS "nb_comments" 
FROM "django_fortunes_fortune" 
LEFT OUTER JOIN "django_comments" 
    ON ("django_fortunes_fortune"."id" = "django_comments"."object_pk") 
GROUP BY "django_fortunes_fortune"."id", 
         "django_fortunes_fortune"."author", 
         "django_fortunes_fortune"."title", 
         "django_fortunes_fortune"."slug", 
         "django_fortunes_fortune"."content", 
         "django_fortunes_fortune"."pub_date", 
         "django_fortunes_fortune"."votes" 
LIMIT 21

你能找出问题所在吗？Django不会使用content_type数据（其中包含对fortune的引用）来左连接django_comments表。在

这是我希望能够使用ORM生成的查询：

SELECT "django_fortunes_fortune"."id", 
       "django_fortunes_fortune"."author", 
       "django_fortunes_fortune"."title", 
       COUNT("django_comments"."id") AS "nb_comments" 
FROM "django_fortunes_fortune" 
    LEFT OUTER JOIN "django_comments" 
        ON ("django_fortunes_fortune"."id" = "django_comments"."object_pk") 
    LEFT OUTER JOIN "django_content_type" 
        ON ("django_comments"."content_type_id" = "django_content_type"."id") 
GROUP BY "django_fortunes_fortune"."id", 
         "django_fortunes_fortune"."author", 
         "django_fortunes_fortune"."title", 
         "django_fortunes_fortune"."slug", 
         "django_fortunes_fortune"."content", 
         "django_fortunes_fortune"."pub_date", 
         "django_fortunes_fortune"."votes" 
LIMIT 21

但我做不到，所以我非常感谢Django退伍军人的帮助：）

提示：我使用的是django1.2-DEV

提前谢谢你的帮助。在

Answer 1

这是Django中的一个bug，请参见罚单#10870。您必须至少等待django1.3或者自己修复它。在

Answer 2

http://charlesleifer.com/blog/generating-aggregate-data-across-generic-relations/http://djangosnippets.org/snippets/2034/

更新： http://github.com/coleifer/django-generic-aggregation/

Answer 3

为什么要加入内容类型表？最初的查询是错误的，但不是因为这个原因。对内容类型的引用是为了标识注释与哪个目标模型相关联，但是您已经知道了，因为您只选择了与Fortune关联的注释。Django已经查询了ContentType模型以获得Fortune的值，因此只需添加一个WHERE子句：

... WHERE django_comments_comment.content_type_id = xx

您可以通过添加与以下内容等效的ORM来正常工作：

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尽管Django并没有自动完成这项工作，这似乎是一个bug。在